1. Field of the Invention
This application relates to systems and methods for measurement of a variety of parameters, e.g., strain, temperature, pressure, etc. More particularly, it concerns such systems and methods that comprise unique combinations of optical waveguides, e.g., optical fibers, operated in multi-mode to generate modal interference patterns and neural networks to interpret such patterns to obtain a value for the parameter being measured.
2. Description of the Prior Art
The use of optical fibers for the sensing of strain is well-known and this has been done in a variety of applications, e.g., see U.S. Pat. Nos. 4,295,738; 4,611,378; 4,653,906 and 4,947,693.
Several different classes of optical sensors have been investigated, each having particular advantages and disadvantages. One class of particular interest is the "few-mode" sensor in which an optical fiber is used that will support two or more lower order propagation modes. As the fiber is subjected to strain, the effective propagation constant for each of the modes is altered in such a way that the relative phase between each mode is shifted a different amount in proportion to the strain. Thus, at the output end of the fiber, all propagating modes interfere, producing an intensity pattern in space which varies with the induced strain. If the fiber is constructed so only the two lowest order modes propagate therein, the interference of these results in two intensity lobes at the output. The light intensity is measured by a photodetector within the spatial illumination area of one of the lobes. As the applied strain is increased, the intensity pattern alternates through successive light/dark transitions (fringes) producing a sinusoidal output signal from the photodetector. To process the detected signal, the modes must be constrained not to rotate with respect to each other. If this is not done, for example in the case of a sensor made from standard circular core fiber, at higher strain levels the intensity pattern rotates so an elliptical core fiber is used to prevent such pattern rotation. Thus, the usual two mode sensor has limited dynamic range and requires special and expensive optical fibers to operate.
If one looks at the other extreme, i.e., the use of standard, low cost, multimode optical fiber where hundreds or thousands of modes propagate, the interference pattern, usually referred to as a speckle pattern, is composed of a very intricate and complicated intensity distribution. In this case the pattern is more sensitive to strain and other parameters (e.g., temperature) as compared with the case when only two modes propagate producing patterns that do not repeat over a large range of strain values. However, it is not easy to process the latter output signal to make full use of the sensitivity and dynamic range possible with the N-mode optical fiber sensors. One possibility would be laboriously to store the intensity distribution data for each strain value. To determine an unknown strain value one could then use image processing techniques to correlate stored images with the one produced from the unknown amount of strain thus identifying the unknown strain value. A serious limitation of this technique is that very high resolution imaging systems are required consequently demanding increased computer processing time to obtain a result. This might require a very expensive system where only static (vs. dynamic strain) might be able to be processed due to the long processing time required for segments of the dynamic waveform. The present invention provides new practical systems and methods for making full use of the sensitivity and dynamic range possible with the N-mode optical fiber sensors in measurement of strain, temperature and other parameter values.